Recently, in order to reduce an environmental load, a thermoelectric conversion technique to effectively use energy has attracted attention. Thus, a high-performance thermoelectric material using a rare metal such as BiTe, PbTe, or SiGe has been conventionally developed as a thermoelectric material to be employed in the thermoelectric conversion technique using the Seebeck effect. In this case, however, since the rare metal is used, there is a problem in view of the environmental load and a risk in resources.
When thermoelectric conversion performance is to be evaluated, dimensionless performance index ZT (=S2σT/k) is used in general. S represents the Seebeck coefficient, σ represents electric conductivity, k represents thermal conductivity, and T represents absolute temperature. The larger the performance index ZT is, the better the thermoelectric conversion performance is. As can be seen from the formula representing the performance index ZT, in order to improve the thermoelectric conversion performance, it is preferable to use a thermoelectric material in which the Seebeck coefficient S and the electric conductivity σ are large, and the thermal conductivity k is small.
In order to solve the problem generated due to the use of the rare metal, it is preferable to use a thermoelectric material made of a ubiquitous element typified by Si. In the case of Si, however, the problem is that the thermal conductivity k is also large although Seebeck coefficient S and the electric conductivity σ are sufficiently large.
Meanwhile, it is reported that the use of a material having a nanostructure as the thermoelectric material leads to the reduction of the thermal conductivity k, and further that the use of a material having a low-dimensional nanostructure leads to a quantum effect and thus to the increase of an index called power factor (S2σ) (non-patent documents 1 to 3).
Thus, studies have been conducted to develop the high-performance thermoelectric material using the nanostructure such as nanowire, nanocomposite, or nanoporous material (non-patent document 4 to 8).
Furthermore, it is reported that the use of the material having a nanodot structure leads to the reduction of the thermal conductivity (non-patent document 9). Some attempts are made such that a nano-opening is formed in an ultrathin silicon oxide film formed on the silicon substrate, and then a nanodot island is epitaxially grown thereon to use as an optical device (non-patent documents 10 to 12). Furthermore, some attempts are made such that by use of the Stranski-Kranstranov (SK) growth, SK dot superlattice is epitaxially grown.
Similarly, patent document 1 discloses a method for producing a semiconductor optical device in which nanodots made of silicon based compound are epitaxially laminated with a space among them filled with a spacer layer made of material such as Si. Patent document 1 also implies that the device is used as a thermoelectric conversion device (paragraph [0042] etc.).